1. Technical Field of the Invention
The present invention relates to gas sensors for detecting a concentration of a specified gas component in measuring gases of, for instance, automotive engines or the like and, more particularly, to a gas sensor having a cover body structure for protecting a gas sensing element.
2. Description of the Related Art
With the development of automotive engines, attempts have heretofore been made to provide a gas sensor mounted on an exhaust pipe of an internal combustion engine such as an automotive engine. The gas sensor detects a concentration of, for instance, a specified gas component such as oxygen in measuring gases to output a detection signal. The detection signal is applied to an electronic control unit, which calculates an air-fuel ratio based on the detected oxygen concentration for thereby performing combustion control of the internal combustion engine.
With such a structure, the gas sensor generally includes a housing fixedly installed on a wall of an exhaust gas flow passage, a gas sensing element inserted through the housing and held in a fixed place with a leading end portion exposed to the exhaust gas flow passage, and a cover body structure covering the leading end portion of the gas sensing element to protect the same from exhaust gases.
Meanwhile, during a startup operation of the internal combustion engine at a low temperature, the exhaust pipe remaining under a cold condition absorbs a heat of moisture, contained in exhaust gases, which in turn is condensed to form water droplets. The water droplets travel through the exhaust gas flow passage. Then, no evaporation of the water droplets takes place and the water droplets intrude an inside of the gas sensor together with measuring gases. Thus, the water droplets tend to adhere onto the gas sensing element with a resultant adverse affect as described below.
In general practice, the gas sensing element, made of a solid electrolyte or the like, is heated with a heater or the like to a high temperature greater than 400° C. to be maintained in an activated state.
Therefore, as the water droplets enters the inside of the gas sensor and adhere onto the gas sensing element, there is a fear of the gas sensing element subjected to a thermal impact to cause a cracking to take place due to water-incursion.
Further, with a view to performing combustion control of the internal combustion engine with increased precision, there is a need for the gas sensor to have a further increased response. Therefore, for the gas sensor to have the increased response, the gas sensor needs to have a structure to immediately admit measuring gases to the inside of the gas sensor.
Accordingly, the cover body of the gas sensor is required to have antimony characteristics with water-incursion resistance and high response.
To satisfy such requirement, attempt has heretofore been made to provide a gas sensor including a cover body formed in a double-layered cylindrical structure composed of an inner shell and an outer shell that are different in diameter from each other as disclosed in Japanese Patent Application Publication 2004-245103. With such a cover body, the inner shell and the outer shell have gas induction holes, respectively, for admitting measuring gases to an inside of the gas sensor so as to ensure a response. The cover body has a sideways clearance defined in a fixed range between an outer periphery of the inner shell and an inner wall of the outer shell, thereby attempting to prevent water droplets from intruding from the sided area of the cover body so as to minimize the occurrence of water-incursion.
In addition, an outer-shell bottom wall opening, formed on an outer-shell bottom wall, and an inner-shell bottom wall opening, formed on an inner-shell bottom wall, are located in a concentric relation to each other, with the inner-shell bottom wall opening being directly exposed to the exhaust gas stream passing through the exhaust gas flow passage.
Further, the present inventors have proposed a gas sensor including a cover body formed in a structure to provide increased water-incursion resistance as disclosed in Japanese Patent Application Ser. No. 2006-124074. An example of a gas sensor with such a structure is shown in FIG. 12.
In FIG. 12, the gas sensor 1C includes a gas sensing element 11 and a cover body formed in a double-layered cylinder structure.
The cover body includes an inner shell 12C and an outer shell 13C, with the outer shell 13C having outer-shell sidewall openings 132C while the inner shell 12C has inner-shell sidewall openings 123C formed in an area axial far from the outer-shell sidewall openings 132C in a direction closer to an inner-shell bottom wall. In addition the outer shell 13C and the inner shell 12C have bottom wall openings 133C and 126C, respectively, formed in a concentric position.
The inner-shell sidewall openings 123C are formed so as to open upward in a direction from an external area of the inner shell 12C to an inside thereof in an upward component. This prevents the water droplets, incoming with the exhaust gas stream through the outer-shell sidewall openings 132C, from entering the inside of the inner shell 12C.
The water droplets in exhaust gases move downward along an inner wall of a reduced diameter portion 124C, formed on the inner shell 12C at a leading end portion thereof, upon which the water droplets are expelled through the bottom wall openings 133C to the measuring gas flow passage.
The bottom wall opening 126C, formed on a bottom wall 125C of the inner shell 12C, is placed on the same plane as the bottom wall opening 133C formed on a bottom wall 135C of the outer shell 13C or a position protruding downward from the bottom wall opening 133C to be directly exposed the measuring gas flow passage.
However, with the structure of the gas sensor disclosed in the related art mentioned above and the structure of the gas sensor shown in FIG. 12, the bottom wall opening, formed on the bottom wall of the inner shell, is exposed to the exhaust gas flow passage, it becomes hard for the gas sensor to completely prevent the water droplets, contained in exhaust gases, from intruding through the inner-shell bottom wall opening depending on an angle at which the gas sensor is mounted on the wall of the exhaust pipe. Especially, under a circumstance where the bottom wall opening 126C, formed on the bottom wall 125C of the inner shell 12C, is placed on the same plane as the bottom wall opening 133C formed on the bottom wall 135C of the outer shell 13C, not only the water droplets, contained in exhaust gases, directly intrude the inside of the inner shell 12C but also the water droplets, remaining on an opening edge of the outer-shell bottom wall opening 133C, are caused to scatter when an exhaust gas stream flows at a high velocity. Thus, there is a fear of the water droplets intruding the inside of the inner shell 12C to adhere onto the gas sensing element.